THE VENTRAL RAMI

The larger ventral rami supply the hypaxial muscles, including those of the limbs (excepting the thoracic girdle muscles supplied by the eleventh cranial nerve and the rhomboideus supplied in some species by dorsal rami) and the remaining skin of the neck, trunk, and limbs.

The Cervical Ventral Rami

The cutaneous distribution of the first two cervical ventral rami extends to the external ear and the mas­seteric and throat regions. The more caudal members of the series contribute to the phrenic nerve and brachial plexus while retaining local responsibilities.

In domestic species the phrenic nerve is generally formed by the fifth, sixth, and seventh cervical nerves. The contributions run ventrally over the scalenus muscle to join in a trunk (see Figure 1-38) that winds below the muscle to enter the mediastinum between the two first ribs. The phrenic nerve runs caudally within the mediastinum, crossing the lateral face of the pericar­dium, to reach the diaphragm; the right nerve utilizes the plica venae cavae in the last part of its course (see Figure 13-15/7). The phrenic nerves ramify within the diaphragm to which they are the sole motor innerva­tion; their sensory fibers are supplemented by others channeled through intercostal nerves. It is worth empha­sizing that the phrenic nerves are typical muscle nerves; it must not be inferred from the normally involuntary nature of breathing that they are autonomic. Experi­ments (in several species) have shown that bilateral section of the phrenic nerves has little effect, although respiratory embarrassment may become evident when the animal is severely stressed.

Figure 8-71 The nerves of the right forelimb of the dog; medial view. 1, Suprascapular n.; 2, subscapular nn.; 3, cranial pectoral nn.; 4, musculocutaneous n.; 4', proximal muscular branch; 4", distal muscular branch; 4"'_l medial cutaneous antebrachial n.; 5, median n.; 6, axillary n.; 7, radial n.; 7’, muscular branches to triceps; 7", muscular branches to exten­sors; 7'", cranial cutaneous antebrachial n.; 8, ulnar n.; 8', caudal cutaneous antebrachial n.; 9, long thoracic n.; 9', tho­racodorsal n.; 9", lateral thoracic n.; 9'", caudal pectoral n.

The Brachial Plexus

The brachial plexus supplies almost all structures of the forelimb—except for the trapezius, omotransversarius, brachiocephalicus, and rhomboideus—and the skin over the upper shoulder region.

The plexus is usually formed by contributions from the last three cervical and first two thoracic nerves; the fifth cervical nerve sometimes participates, and the con­tribution of the second thoracic nerve is then reduced or lacking. The plexus reaches the axilla by passing between the parts of the scalenus and quickly splits into peripheral branches that diverge toward their separate destinations (see Figure 8-71). Several branches have very restricted local distributions, and bare mention of their names and destinations is all that is required; they comprise the long thoracic nerve (Figure 8-71/9) to the serratus ventralis, the thoracodorsal nerve (Figure 8-71/9') to the latissimus dorsi, cranial and caudal pec­toral nerves (Figure 8-71/5,9'") to the pectoral muscles (including the subclavius), the subscapular nerve (Figure 8-71/2) to the subscapularis, and the lateral thoracic nerve (Figure 8-71/9") to the cutaneous trunci and to skin over the ventral part of the thorax and abdomen. The other branches require fuller description. Though some exhibit interspecies differences, these are rarely of importance except in the manus, and even these will be largely disregarded in the meantime.

The suprascapular nerve (Figure 8-71/1) leaves the cranial part of the brachial plexus (C6-C7). It passes between the supraspinatus and subscapularis to reach the cranial margin of the neck of the scapula, around which it winds to the lateral aspect of the bone, where it is expended within the supraspinatus and infraspina­tus muscles. Like other nerves directly related to bone, it is vulnerable to injury; in this case it is usually stretched against the scapula when the limb is overabducted or violently retracted. The resulting paralysis of the lateral shoulder muscles does not affect the standing posture but may result in an obvious lateral movement of the shoulder joint (“shoulder slip”) during the stride. The condition occurs most frequently in horses, in which it is also known as “sweeny”; it manifests itself after a time by obvious wasting of the muscles beside the scap­ular spine.

The musculocutaneous nerve (Figure 8-71/4) is also of cervical origin (C7-C8). After a short course within the axilla the nerve branches off the proximal muscular branch (Figure 8-71/4'), which supplies the coracobra­chialis and biceps in the upper part of the arm. In the dog the continuation beyond the proximal muscular branch remains separate from the median nerve until in the distal third of the arm a communicating branch passes distocaudally to the median nerve. The continu­ing nerve passes under the terminal part of the biceps brachii, where it divides into the distal muscular branch (Figure 8-71/4"), which supplies the brachialis, and the medial cutaneous nerve of the forearm (Figure 8-71/4'"), which crosses the flexor aspect of the elbow before rami­fying in skin.

In ungulates generally, the musculocutaneous nerve loops around the axillary artery to join the median nerve in which its identity is for a time submerged; the musculocutaneous fibers again separate from the median nerve in the upper and lower parts of the arm, where they form the proximal and distal muscular branches of the musculocutaneous nerve.

Section of the main musculocutaneous trunk is an unlikely injury; it would paralyze the main flexors of the elbow, although compensation would probably be found from activity of the carpal and digital extensors.

The axillary nerve (C8) (Figure 8-71/6) passes behind the shoulder joint to reach the lateral aspect of the limb. En route it supplies the teres major, teres minor, capsu­laris, and deltoideus—the true flexors of the shoulder joint. It also supplies twigs to the distal part of the brachiocephalicus, which, it will be recalled, is of deltoid origin. A cutaneous branch supplies skin over the cranial aspect of the arm and forearm.

The three remaining branches of the plexus have the most complicated courses and the most extensive distri­butions. The radial nerve (Figure 8-71/7) arises from the last two cervical and first thoracic nerves (C7-T1). It first runs distally within the arm, caudal to the brachial artery, before diving between the long and medial heads of the triceps to follow the spiral groove of the humerus, which leads it to the craniolateral aspect of the limb. While buried by the triceps, it supplies branches to the various heads of this muscle (Figure 8-71/7') and to tensor fasciae antebrachii and anconeus. In the lower part of the arm the radial nerve supplies a further set of muscular branches (Figure 8-71/7") to all carpal and digital extensor muscles, including the anomalous ulnaris lateralis. A cutaneous branch (Figure 8-71/7'"), often replicated, descends over the craniolateral aspect of the forearm and carpus to reach the dorsal surface of the digits, except in the horse, in which it fades about the level of the carpus because part of the more distal duty is assumed by the musculocutaneous nerve.

Damage to the radial nerve can have three obvious consequences: paralysis of the elbow extensors, paraly­sis of the carpal and digital extensors, and anesthesia of the skin territory.

The median nerve (Figure 8-71/5) comes mainly from the last cervical and first thoracic nerves (C8-T1). It runs down the medial surface of the arm caudal to the main artery and enters the forearm over the medial col­lateral ligament of the elbow joint. It then inclines cau- dally, passes under the flexor carpi radialis, and maintains this protected situation until it reaches the carpus. It divides in the distal part of the forearm, or within the carpal canal, into two or more divisions that descend through the carpal canal to supply most struc­tures of the palmar part of the foot. The median nerve supplies most of the flexor muscles of the carpus and digit in a pattern that overlaps (but does not quite coin­cide) with the distribution of the ulnar. Because of this, damage confined to the median nerve does not usually manifest itself through any abnormality of posture or gait.

The ulnar nerve (Figure 8-71/8) leaves the caudal part of the plexus (C8-T2). It runs down the arm beside and possibly (as in the dog) for a stretch united to the median nerve before deviating in the direction of the olecranon to cross the caudal aspect of the elbow joint.

Damage confined to this nerve is unlikely to impair locomotion; the sensory deficits show considerable interspecies variation.

The Thoracic Ventral Rami

These show a more strictly segmental distribution than is found in other regions. The first two contribute to the brachial plexus, but generally the thoracic ventral rami provide the intercostal nerves that run ventrally within the intercostal spaces, either directly below the pleura or between the two intercostal muscle layers; the rela­tion varies according to location and species. Apart from supplying the intercostal muscles, the intercostal nerves detach lateral cutaneous branches that supply a band of skin over the lateral aspect and ventral cuta­neous branches that supply the ventral aspect of the chest wall; the more caudal members of the series are also concerned in the supply of the abdominal floor. There are a few minor connections with nerves of the brachial plexus. In the sow, bitch, and cat the lateral cutaneous branches detach twigs to thoracic mammary glands.

The last thoracic ventral branch (costoabdominal nerve) is slightly different in its course and distribution because it runs behind the last rib. It collaborates with lumbar ventral branches in the supply of the flank.

The Lumbar Ventral Rami

The lumbar and sacral ventral rami form a continuous plexus, best developed where the last three or four lumbar and first two sacral nerves form the lumbosacral plexus that supplies the hindlimb. The more cranial lumbar ventral rami have a considerable importance in cattle because they are frequently blocked for abdomi­nal surgery. They are given individual names; in species (including cattle) in which there are six lumbar nerves, the first ventral ramus is known as the iliohypogastric, the second is known as the ilioinguinal, and the third and fourth combine to form the genitofemoral nerve. In species with seven lumbar nerves the first two ventral rami are distinguished as the cranial and caudal iliohy­pogastric; the third supplies the ilioinguinal and also makes a contribution to the genitofemoral nerve. The genitofemoral nerve divides into a femoral branch that supplies the skin over the medial aspect of the thigh and a genital branch that supplies the spermatic fasciae, the scrotum, and the prepuce.

The caudoventral inclination of the ventral rami that becomes increasingly apparent with the caudal intercos­tal nerves is further accentuated with the lumbar rami; the locations where the nerves can most easily be reached by injection of local anesthetic solution and the posi­tions of their dermatomes are both considerably more caudal than would naturally be supposed (see Figure 28-2). The nerves pass through the transversus close to the tip of the transverse processes and then run deep to the internal oblique toward the abdominal floor (see Figure 1-37). In addition to supplying the flank and rectus muscles, they detach lateral and ventral cutane­ous branches; the former appear subcutaneously at increasingly dorsal levels as the series is followed caudally.

The Lumbosacral Plexus

The lumbosacral plexus that gives origin to the nerves of the hindlimb (with the minor exceptions of those to certain proximal skin areas) is an enhancement of the continuous plexus. It usually begins with the ventral

Figure 8-72 The lumbar and sacral nerves of the dog; medial view. 1, Femoral n.; 1', branches to quadriceps; 1", saphenous n.; 2, obturator n.; 3, pelvic n.; 4, branch to obtura­tor internus, gemelli, and quadratus femoris; 5, sciatic n.; 6, peroneal n.; 6', lateral cutaneous sural n.; 6", superficial pero­neal n.; 6'", deep peroneal n.; 7, tibial n.; 7, caudal cutaneous sural n.; 7", medial plantar n.; 7'", lateral plantar n.; 8, puden­dal n.; 8', deep perineal n.; 9, caudal cutaneous femoral n.; 10, caudal rectal n.

ramus of the fourth lumbar nerve and ends with that of the second sacral (L4-S2); it thus has an additional root in species possessing seven lumbar nerves (Figure 8-72).

The femoral nerve (Figure 8-72/1) arises from the cranial part (L4-L6) of the plexus and pursues a course through the psoas muscles to reach the gap between the dorsocaudal corner of the flank and the iliopsoas muscle. It is accompanied by the external iliac artery and vein, and on entering the thigh it runs in a protected position between the sartorius and pectineus. It soon detaches the saphenous nerve, and after a very short further course it dives between the rectus femoris and vastus medialis to be expended within the quadriceps mass (Figure 8-72/1 ). Severe damage to this nerve, though relatively infrequent, has serious consequences because paralysis of the quadriceps precludes fixation of the stifle joint, which renders the whole limb inca­pable of supporting weight. No compensation for this defect is possible.

The saphenous nerve (Figure 8-72/1") gives a branch to the sartorius before continuing to supply skin over the medial aspect of the limb from the stifle to the metatarsus.

The obturator nerve (Figure 8-72/2) has broadly the same origin (L4-L6) as the femoral nerve. It follows the medial aspect of the shaft of the ilium to reach the obturator foramen through which it passes to the adductor muscles of the thigh; the group comprises gracilis, pectineus, adductor, and obturator externus— and obturator internus in ruminants and the pig. The relationship to bone is potentially dangerous, exposing the nerve to the risk of laceration in fractures and to the risk of compression during calving and foaling (the risk is less in species in which the young are small rela­tive to the pelvic cavity). The effects of injury vary with its extent but are greater in heavier animals and are exaggerated by a requirement to walk on smooth ground, when the limb tends to slip sideways.

The remaining branches of the plexus arise from a common lumbosacral trunk that is largely formed by the last lumbar and first two sacral nerves, along with a smaller contribution from the penultimate lumbar nerve. The trunk leaves the pelvis through the greater sciatic foramen and almost at once detaches three branches.

The short cranial gluteal nerve supplies the tensor fasciae latae, the middle and deep, and in some species part of the superficial gluteal muscles, a group that— contrary to the usual expectation—includes both flexor and extensor muscles of the hip.

The caudal gluteal nerve supplies the superficial gluteal muscle and the vertebral heads of origin of the hamstring muscles (biceps femoris, semitendinosus, and semimembranosus), parts supposed to represent assimi­lation of elements of the superficial gluteal. It thus supplies extensor muscles of the hip.

The caudal cutaneous femoral nerve (8-72/9) supplies skin over the caudal aspect of the thigh.

The sciatic nerve (Figure 8-72/5) continues the lum­bosacral trunk distally, passing between the middle and deep gluteal muscles before turning into the thigh caudal to the hip joint, where it is protected by the greater trochanter of the femur. It then runs between the biceps femoris laterally and the semitendinosus medially before dividing into its terminal branches, the common peroneal and tibial nerves, at a level that varies among species. In the proximal part of its course it detaches twigs to the unimportant internal obturator (except in ruminants and pigs), gemelli, and quadratus femoris (Figure 8-72/4); other muscular branches that may appear to arise directly from the sciatic nerve are usually referred to its common peroneal and tibial divisions.

The common peroneal nerve (Figure 8-72/6), the lesser of the terminal branches, arises from the lumbar roots of the lumbosacral trunk. It runs first with the tibial nerve but separates from this to pass over the lateral head of the gastrocnemius to enter the leg. It detaches a branch, the lateral sural nerve (Figure 8-72/6'), to the skin over the lateral aspect of the leg before dividing into superficial and deep branches when close to the head of the fibula. The superficial peroneal nerve (Figure 8-72/6") supplies skin over the dorsal aspect of the leg and entire foot, except in the horse, in which it fades about the level of the fetlock joint. The deep peroneal nerve (Figure 8-72/6'") supplies the dor­solateral muscles of the leg (flexors of the hock and extensors of the digits) and is also sensory to the struc­tures of the foot. Because the sensory innervation of pedal structures varies considerably, the details are deferred until the accounts of individual species.

Paralysis of the common peroneal nerve produces overextension of the hock and flexion of the digits, which may be rested on their dorsal surfaces. The foot may be passively placed to support weight, and in time compensation may be possible (cf. radial paralysis, p. 322). There is also a considerable sensory deficit.

The tibial nerve (Figure 8-72/7) arises from the sacral roots of the lumbosacral trunk. It detaches important proximal muscular branches to the pelvic heads of the hamstring muscles before freeing itself from the parent trunk to enter the leg by passing between the two heads of the gastrocnemius. About this level it first detaches a caudal sural nerve (Figure 8-72/7') to the skin of this aspect of the leg and later detaches distal muscular branches to the gastrocnemius, soleus, popliteus, and caudal crural muscles. The nerve continues as an almost exclusively sensory trunk (although it will supply short digital muscles) within the fascial plate between the common calcanean tendon and the caudal crural muscles; it ends by dividing into medial and lateral plantar nerves when level with the point of the hock. The plantar nerves (Figure 8-72/7", 7'") continue into the plantar aspect of the foot to supply sensation to plantar structures chiefly but with some dorsal penetration that varies among species.

Section or severe damage to the tibial nerve is mani­fested by overflexion of the hock and overextension of the digits. Similar damage to the parent trunk combines the effects of common peroneal and tibial nerve inju­ries, rendering the limb largely incapable, although fixa­tion of the stifle joint by the unaffected quadriceps may allow it to support some weight.

The Sacral and Caudal Ventral Rami

The sacral ventral rami caudal to and overlapping the roots of the lumbosacral plexus give rise to other impor­tant individual nerves. The pelvic nerves (Figure 8-72/5) composed of the parasympathetic outflow are consid­ered in the following section.

The pudendal nerve (Figure 8-72/8) arises variously (S1-S3 in the dog, S2-S4 in ruminants, S[2]3-S4 in the horse). It is sensory to the rectum, internal and external reproductive organs, and perineal skin and motor to much of the striated perineal musculature. It has both physiological and applied importance, but because it is variable, it must for the present suffice to say that its course takes it obliquely through the pelvis toward the ventral part of the outlet (see Figure 29-5/7). It provides deep and superficial perineal nerves in addition to various cutaneous branches and finally continues as the dorsal nerve of the penis (or clitoris). The superficial perineal branch supplies the skin of the anus, vulva, and ventral perineal region, the strict perineal location.

The deep perineal nerve supplies the ventral part of the striated musculature of the perineum, particularly that of the reproductive organs. The main trunk also supplies branches to the skin of the prepuce and scrotum in the male and of the caudal part of the udder in the ungulates.

The caudal rectal nerves (Figure 8-72/10) arise from the most caudal sacral nerves, sometimes overlapping the origin of the pudendal nerve. They supply sensory fibers to the rectum, anus, and perianal skin, and motor fibers to the dorsal perineal striated musculature, includ­ing the levator ani. The division of territory between these nerves and the pudendal is rather variable.

The ventral rami of the caudal nerves supply the ventral or depressor muscles of the tail.